Analog cellular telephone systems, such as AMPS, ETACS, NMT-450, and NMT-900, have long been deployed successfully throughout the world. Since the early 1990's, digital cellular telephone systems such as those conforming to the North American standard IS-54 and the European Global System for Mobile Communications (“GSM”) standard have been in service. More recently, a wide variety of wireless digital services broadly labeled as PCS (Personal Communications Services) have been introduced, including advanced digital cellular systems conforming to standards such as IS-136 and IS-95, lower-power systems such as DECT (Digital Enhanced Cordless Telephone) and data communications services such as CDPD (Cellular Digital Packet Data). These and other systems are described in The Mobile Communications Handbook, edited by Gibson and published by CRC Press (1996).
In most cellular radiotelephone systems, the cellular system sets up a distinct, full-duplex, circuit-switched channel for each standard voice telephone call that is carried by the system. Depending upon the cellular system, this circuit-switched channel may be implemented, for example, as an FDMA channel, a TDMA channel, a TDMA/CDMA channel, etc. The circuit-switched channel is established by the cellular network at the time the call is initiated and is maintained until the call is terminated.
In recent years, various cellular telephone systems have implemented capabilities for supporting additional types of communications. By way of example, cellular systems that operate under the GSM standard may provide various packet-switched communications services that may be implemented according to the General Packet Radio Service (“GPRS”) standards that are part of the GSM standard. Such packet-switched communications services may include e-mail, web-browsing, instant messaging and the like. Another communication capability that has received increased interest in recent years is a “push-to-talk” capability. Push-to-talk refers to a “walkie-talkie” like feature where two users (point-to-point talk) or a larger group of users (group-talk) may engage in instant communications. Once a push-to-talk communications session is established, participants in the session typically do not need to dial a phone number to converse with other participants. The communications session may be left open for extended periods of time, even if the participants are in stand-by mode (i.e., not actually conversing). When a participant chooses to converse with another participant of the push-to-talk session, they typically need only push and hold a button on their cell phone (or other wireless terminal) and speak into the microphone.
The Open Mobile Alliance (“OMA”) has established a working group that is in the process of defining a specification under which a push-to-talk capability will be implemented for GSM and other cellular systems. While there are potentially a variety of different ways that such a push-to-talk capability could be implemented, the OMA is defining specifications whereby the push-to-talk capability will be implemented using voice-over-IP resources that are transmitted over GPRS. Voice-over-IP (“VoIP”) refers to a form of packet-switched communications capability that operates over the Internet Protocol (“IP”) to support voice communications. In a VoIP system, voice communications are digitized and transmitted in small data packets over a communications network to the intended recipients. The data packets may travel over different paths through the network and are reassembled into their proper order at the destination so that they may be converted back into an analog voice signal that may be played to the remote user. Under current OMA plans, on the cellular network side, GPRS will be used to implement the VoIP push-to-talk capability. Wireless terminals that implement the push-to-talk capability will, therefore, have both a cellular address and an IP address (URL).
The push-to-talk capability may be implemented as a half-duplex channel that is shared by all of the participants of the push-to-talk session. A wireless terminal user may arrange for establishment of the push-to-talk session by, for example, dialing a number on the keypad of their cellular telephone (or other wireless terminal) that is associated with the remote user to whom they wish to speak. The wireless terminal sends a page to a network server notifying the network server of the request for establishment of a push-to-talk session. The network server establishes a push-to-talk session between the user and the remote user. The push-to-talk session may be established between two users (point-to-point) or between three or more users (group talk). The push to talk session typically is maintained until it is terminated by one of the users.
Once the push-to-talk session has been activated, either participant may send a floor request (a request to use the half-duplex channel) by pushing a button on their wireless terminal. The network server may indicate that the floor request has been granted by, for example, an audio tone. Once the user has been granted the floor he may broadcast speech over the channel by talking while holding in the button. Release of the button acts to release the floor so that the half-duplex channel may then be used by another participant to the call or remain idle. While the push-to-talk session is active, it provides the user and remote user(s) an instant talk capability for communicating with each other.
Under the current plans of the OMA working group, the GPRS push-to-talk transport protocol will use the Session Initiated Protocol (“SIP”) and the Session Description Protocol (“SDP”) for transmission of control and signaling information, and will use the Real Time Protocol (“RTP”) for transmitting data (i.e., the voice traffic). The Real Time Control Protocol (“RTCP”) will be used to transmit floor request/notification signals (i.e., the signals that are sent when a user grabs the channel by pressing the push-to-talk button on the wireless terminal and releases the channel by releasing the button) and the start/end burst packets. The present OMA proposal is to implement the push-to-talk channel using a 4.75 kbps adaptive multi-rate (AMR) coded speech data channel, with other data rate channels to possibly be added later.